Gas discharge sources are used, for example, as light sources for EUV radiation (EUV: extreme ultra violet) or soft x-rays. Radiation sources emitting EUV radiation and/or soft x-rays are in particular required in the field of EUV lithography. The radiation is emitted from hot plasma produced by a pulsed current. The most powerful EUV radiation sources known up to now are operated with metal vapor to generate the required plasma. An example of such a EUV radiation source is shown in WO 2005/025280 A2. In this known radiation source the metal vapor is produced from a metal melt which is applied to a surface in the discharge space and at least partially evaporated by an energy beam, in particular by a laser beam. In a preferred embodiment of this radiation source the two electrodes are rotatably mounted forming electrode wheels which are rotated during operation of the radiation source. The electrode wheels dip during rotation into containers with the metal melt. A pulsed laser beam is directed directly to the surface of one of the electrodes in the discharge region in order to generate the metal vapor from the adhered metal melt and ignite the electrical discharge. The metal vapor is heated by a current of some kA up to some 10 kA so that the desired ionization stages are excited and light of the desired wavelength is emitted. The liquid metal film formed on the outer circumferential surfaces of the electrode wheels serves as the radiating medium in the discharge and protects as a regenerative film the wheel from erosion.
For stable EUV radiation output of such a EUV discharge lamp, it is required that consecutive discharge pulses are hitting always a fresh smooth portion of the electrode surfaces. The distance of consecutive discharge pulses on the moving electrode surface is in the order of a few tenths of millimeter up to a few millimeters. Increasing the power of the lamp is possible mainly by increasing the repetition rate of the discharge. Therefore, the electrode rotational speed must be increased accordingly.
It has been found experimentally, that the film thickness of the liquid metal film on the rotating electrode increases with increasing rotational frequency due to the higher centrifugal forces. At high rotational frequencies, the film thickness can reach several hundreds of microns, resulting in the formation of liquid metal droplets spraying off the electrode surface. These droplets can cause short circuits in the lamp and thus lamp failure. Moreover, a varying film thickness of the liquid metal film influences the effective distance between the electrodes. This requires an optimization of the operational parameters of the lamp for each rotational frequency. WO 2005/025280 A2 discloses the use of strippers or wipers in order to ensure a limited thickness of the liquid material film applied to the outer circumferential surface of the electrode wheels. Nevertheless, the rotational frequency of the electrode wheels is limited due to the formation of droplets or instabilities of the liquid metal film at higher rotational speeds.